Java / Java 21 Interview Questions
What Stream API improvements were introduced in Java 9 through Java 21?
The Stream API has been incrementally enhanced since Java 9. Knowing which methods are available and when they arrived is frequently tested.
| Method | Since | Description |
|---|---|---|
| takeWhile(Predicate) | 9 | Takes elements while predicate is true, then stops |
| dropWhile(Predicate) | 9 | Drops elements while predicate is true, then takes rest |
| iterate(seed, hasNext, next) | 9 | Finite iterate — 3-arg form with a terminating predicate |
| ofNullable(T) | 9 | Stream of 0 or 1 elements (empty if null) |
| Stream.of(T...) | 9 | Already existed; ofNullable is the new addition |
| Collectors.teeing(d1, d2, merger) | 12 | Collect into two downstreams, merge results |
| toList() | 16 | Unmodifiable List directly from stream (vs Collectors.toList()) |
| mapMulti(BiConsumer) | 16 | Flexible flat-map alternative |
| Stream.gather(Gatherer) | 22 preview | Custom intermediate operations (post-21) |
// takeWhile / dropWhile — Java 9
List.of(1, 2, 3, 4, 5, 1, 2)
.stream()
.takeWhile(n -> n < 4) // [1, 2, 3] — stops at first failure
.toList();
// 3-argument iterate — finite sequence
Stream.iterate(1, n -> n <= 100, n -> n * 2) // 1 2 4 8 ... 64
.toList();
// ofNullable — avoids NPE in flatMap chains
Stream.ofNullable(maybeNull) // empty stream if null, else stream of one
.map(String::toUpperCase)
.findFirst();
// Collectors.teeing — Java 12
record Stats(long count, double sum) {}
Stats s = numbers.stream().collect(
Collectors.teeing(
Collectors.counting(),
Collectors.summingDouble(Double::doubleValue),
Stats::new
));
// toList() — Java 16
List names = people.stream().map(Person::name).toList();
// returns an UNMODIFIABLE list — names.add("x") throws UnsupportedOperationException What does Stream.toList() (added in Java 16) return, compared to Collectors.toList()?
What does Stream.ofNullable(value) produce when value is null?
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